Trying to create a Mongo Express container from instructions

Question

I am using the documentation on the mongo-express page in Docker Hub to create my container. I used the command:

$ docker run --network some-network -e ME_CONFIG_MONGODB_SERVER=some-mongo -p 8081:8081 mongo-express

Now I assume that this is supposed to work as is, but I keep getting the error:

Could not connect to database using connectionString: mongodb://some-mongo:27017/"